Hydraulic actuators such as cylinders having an extendable and retractable rod and piston are utilized to accomplish a variety of tasks. These actuators are connected to a pump that provides pressurized fluid to chambers within the actuators. As the pressurized fluid moves into or through the chambers, the pressure of the fluid acts on hydraulic surfaces of the chambers to effect movement of the rod and piston. When the pressurized fluid is drained from the chambers it is returned to a low pressure tank. The speed at which the fluid flows into and out of the chambers affects the extension and retraction speeds of the actuator, while the pressure in contact with the hydraulic surfaces affects the actuation force thereof.
One problem associated with this type of hydraulic arrangement is the energy wasted when pressurized fluid flow is throttled through a valve in order to control movement speed of the actuator. In particular, because the movement speed of the actuator is directly related to the flow rate of fluid entering and leaving the chambers thereof, to move the actuator at a slow speed, the flow of fluid must be throttled to a lower rate than, as compared to high speed movements of the actuator. By throttling the flow of fluid, energy from the fluid is converted to heat, which must then be dissipated to the environment. In this situation, energy that could have been utilized to move the actuator is lost from the fluid, and additional energy must be utilized to remove the heat from the system.
Another problem associated with this type of hydraulic arrangement involves the magnitude of the pressure required to move the actuators, particularly when the actuators are heavily loaded. Specifically, if the actuator is intended to move heavy loads quickly, the pump supplying fluid to the actuators must be sized to provide very high flow rates at relatively high pressures. This requirement increases the size of the pump and, subsequently, the overall cost of the machine.
In addition, when the pressures are significantly high, precise movement control of the actuators can be difficult, and not all actuators powered by a common pump require the same high pressure. As a result, in some situations, the high pressure must be throttled down to meet the requirements of control and low pressure actuators. As described above, throttling of the pressurized fluid is an inefficient way to achieve the desired result.
Further, the efficiency of a system employing such a large pump is low due to unused pressurized fluid being wasted. In particular, the fluid draining from the actuator chambers to the tank often has a pressure greater than the pressure of the fluid already within the tank. In fact, when under load, this pressure can be much greater than the tank pressure. As a result, the higher pressure fluid draining into the tank still contains some energy that is wasted upon entering the low pressure tank. This wasted energy reduces the efficiency of the associated hydraulic system.
One attempt at addressing some of these pressure difficulties is described in U.S. Patent Publication No. 2002/0104313 (the '313 publication) disclosed by Clarke on Aug. 8, 2002. The '313 publication describes a hydraulic transformer that uses a pair of variable displacement gear pumps. The two gear pumps are disposed on a common shaft and receive fluid in parallel from a common high pressure source. The outlet of a first of the gear pumps is fluidly connected to a hydraulic piston, while the second of the gear pumps is fluidly connected to a tank. As the fluid flows through the second gear pump to the tank, energy is removed from the fluid and utilized to turn the common shaft and connected first gear pump. As the fluid flows through the first gear pump, the torque applied to the common shaft by the second gear pump is utilized to increase the pressure of the fluid flowing to the hydraulic piston. In this manner, although the pressure of the fluid supplied to the hydraulic transformer may be less than the pressure required to move the hydraulic piston, the hydraulic transformer accommodates this deficiency by increasing the pressure of a portion of the supplied flow high enough for useful operation. With this configuration, lower pressure actuators may receive flow directly from the source without throttling, while higher pressure actuators may receive flow from the transformer having pressure adequate to move high loads.
Although the hydraulic transformer described in the '313 publication may alleviate the need for throttling or an oversized high-pressure pump, its use may be limited. Specifically, the hydraulic transformer of the '313 publication is only unidirectional, with fluid flowing only from the source to the piston. Because of this limitation, source pressure amplification and energy recuperation from draining chambers of the piston may be impossible with the hydraulic transformer. In addition, because the hydraulic transformer is variable displacement, the cost and complexity of the transformer may be excessive.
The disclosed hydraulic transformer is directed to overcoming one or more of the problems set forth above.